The present invention concerns a watch with a compass function, the watch comprising an electronic assembly including a watch movement and associated with manual control members, and with a magnetic sensor assembly, capable of measuring the direction of the terrestrial magnetic field in relation to the watch, and with display means comprising central hands activated by electric motors to indicate the time on a dial in a time display mode and to indicate the direction of north in a compass mode by means of two of said hands, set and held in mutually aligned opposite positions.
A wristwatch of this type, marketed by Tissot SA under the name T Touch®, is the subject of CH Patent No. 690 697 and includes an analogue display with two hands, in addition to a digital liquid crystal display. The compass includes a magnetic sensor assembly of the type described in EP Patent No. 713 162, having a radially magnetised rotor and two Hall probes arranged in fixed positions. The control members of this multi-function watch include three lateral push-buttons and seven capacitive keys formed by transparent electrodes on the inner face of the watch crystal. In compass mode, the direction of north is continuously indicated by means of the hour and minute hands, aligned opposite each other to form together a single hand long enough to pass over a large part of the dial, thus resembling the magnetised hand of a conventional compass. A notable advantage of this magnetic watch-compass is that it indicates the direction of north in real time, independently of any movements that the user might impart to the watch during use of the compass.
This advantage does not exist in watches or other apparatus which calculate and display a direction from two successive measurements made by a terrestrial satellite positioning system such as GPS, for example as provided in EP Patent No. 871 096. In fact, according to this system, the direction of point selected as the target is indicated in relation to a reference axis of the watch, for example the 6 o'clock–12 o'clock axis of the dial, that the user has to keep in the direction of movement defined by the two preceding measurement points. As this reference system is not fixed in relation to the Earth, any rotational movement of the watch before the measurement of the position of the following point is accompanied by the hand and thus distorts the direction indication.
Moreover, EP Patent No. 713 162 discloses a magnetic compass watch that can indicate, via one of its two hands, a geographical azimuth comprised between 0° and 360°, that the user will have pre-selected beforehand using the manual control means of the watch. It is also provided means for imputing the magnetic declination of the location in order to convert the magnetic azimuths into geographical azimuths. The user can operate his watch such that the hand indicates either north (zero azimuth), or a pre-selected heading, to follow, but not both at the same time.